Electromagnetic clutches and brakes are used extensively in a variety of machines. They are used in stationary machines and on vehicles. Vehicle use includes use in drives for air conditioning system compressors, air compressors, superchargers, and other devices. These clutches can take many forms. Common components of the clutches include a field coil assembly, a rotor assembly and an armature assembly.
The field coil assembly is generally mounted in a fixed position. A rotor assembly is mounted adjacent to the field coil in a position to form a portion of a magnetic flux path. The rotor assembly includes a drive engaging means, such as a v-belt or power band engaging surface, a sprocket for a chain drive or a gear for a gear drive, and an attraction surface. The armature assembly includes an armature hub, an armature and connecting members which connect the armature to the armature hub. The connecting members allow the armature to move axially relative to the armature hub to engage the attraction surface on the rotor assembly and to move out of engagement with the attraction surface. The connecting members can also cushion the high peak torque that occurs when the armature contacts the attraction surface.
The armature hub has a splined bore. The bore receives a splined shaft. When the clutch is used to drive a compressor, the splined shaft is the driven shaft. The armature hub can be axially fixed on the driven shaft by a surface on the rear of the armature hub adjacent to the splined bore contacting a surface on the splined shaft at a point where the diameter of the splined shaft increases. The armature hub can also be axially fixed on the driven shaft by an inside surface of a cap partially closing the front of the splined bore through the armature hub contacting the end of the splined shaft. The hub is retained on the shaft by a bolt which screws into a threaded bore in the end of the splined shaft and holds the armature hub in an axially fixed position relative to the splined shaft.
The axial position of the armature hub on the splined shaft fixes the space between the armature and the attraction surface when the field coil is not energized. This space is referred to as the air gap. The air gap is critical. If there is no air gap the clutch is engaged when the electromagnetic coil is deenergized as well as when it is energized. If the air gap is too wide, the electromagnetic force produced by the energized field coil may be insufficient to pull the armature axially into contact with the attraction surface. The air gap is especially critical when the field coil is at an elevated temperature.
The air gap can be adjusted by placing washer-shaped spacer shims on the shaft between a surface on the rear portion of the armature hub and a portion of the splined shaft with increased diameter. The air gap can also be adjusted by placing washer-shaped shims in the splined bore in the armature hub between the end of the splined shaft and the inside surface of a cap partially closing the forward end of the splined bore. The air gap can be accurately adjusted by employing the shims described above. However, it can be time consuming to measure the air gap, remove or add washer-shaped shims as required and remeasure the air gap.
To reduce manufacturing costs, some electromagnetic clutches are assembled by machines. The electromagnetic clutch assembly machines currently used have difficulty with washer-shaped shims. The machines fail to insert a shim that is required or shims stick together and too many shims are inserted. As a result the air gap tolerance range is too broad. The broad tolerance range can potentially cause premature failure of the clutch.